Use PyContracts so we can declare/enforce parameter and return types.

This commit doesn't add any uses of PyContracts, but gets the machinery in
place.

3 files changed, 41 insertions, 6 deletions

diff --git a/coverage/control.py b/coverage/control.py
index 2c8d384e..defba56c 100644
--- a/coverage/control.py
+++ b/coverage/control.py
@@ -169,7 +169,7 @@ class Coverage(object):
         self.source_pkgs = self.file_locator = None
         self.data = self.collector = None
         self.plugins = self.file_tracing_plugins = None
-        self.pylib_dirs = self.cover_dir = None
+        self.pylib_dirs = self.cover_dirs = None
         self.data_suffix = self.run_suffix = None
         self._exclude_re = None
         self.debug = None
@@ -301,7 +301,12 @@ class Coverage(object):
 
         # To avoid tracing the coverage code itself, we skip anything located
         # where we are.
-        self.cover_dir = self._canonical_dir(__file__)
+        self.cover_dirs = [self._canonical_dir(__file__)]
+        if env.TESTING:
+            # When testing, we use PyContracts, which should be considered
+            # part of coverage.
+            import contracts
+            self.cover_dirs.append(self._canonical_dir(contracts))
 
         # Set the reporting precision.
         Numbers.set_precision(self.config.precision)
@@ -315,8 +320,8 @@ class Coverage(object):
             self.source_match = TreeMatcher(self.source)
             self.source_pkgs_match = ModuleMatcher(self.source_pkgs)
         else:
-            if self.cover_dir:
-                self.cover_match = TreeMatcher([self.cover_dir])
+            if self.cover_dirs:
+                self.cover_match = TreeMatcher(self.cover_dirs)
             if self.pylib_dirs:
                 self.pylib_match = TreeMatcher(self.pylib_dirs)
         if self.include:
@@ -1033,7 +1038,7 @@ class Coverage(object):
         info = [
             ('version', covmod.__version__),
             ('coverage', covmod.__file__),
-            ('cover_dir', self.cover_dir),
+            ('cover_dirs', self.cover_dirs),
             ('pylib_dirs', self.pylib_dirs),
             ('tracer', self.collector.tracer_name()),
             ('file_tracing_plugins', ft_plugins),
diff --git a/coverage/env.py b/coverage/env.py
index 85ffa5ff..af78844e 100644
--- a/coverage/env.py
+++ b/coverage/env.py
@@ -15,7 +15,12 @@ PY2 = sys.version_info < (3, 0)
 PY3 = sys.version_info >= (3, 0)
 
 # Coverage.py specifics.
+
 # Are we using the C-implemented trace function?
 C_TRACER = os.getenv('COVERAGE_TEST_TRACER', 'c') == 'c'
+
 # Are we coverage-measuring ourselves?
 METACOV = os.getenv('COVERAGE_COVERAGE', '') != ''
+
+# Are we running our test suite?
+TESTING = os.getenv('COVERAGE_TESTING', '') != ''
diff --git a/coverage/misc.py b/coverage/misc.py
index d5197ea3..64111358 100644
--- a/coverage/misc.py
+++ b/coverage/misc.py
@@ -6,7 +6,32 @@ import inspect
 import os
 
 from coverage import env
-from coverage.backward import string_class, to_bytes
+from coverage.backward import string_class, to_bytes, unicode_class
+
+
+# Use PyContracts for assertion testing on parameters and returns, but only if
+# we are running our own test suite.
+contract = None
+
+if env.TESTING:
+    try:
+        from contracts import contract
+    except ImportError:
+        pass
+    else:
+        from contracts import new_contract
+
+        # Define contract words that PyContract doesn't have.
+        new_contract('bytes', lambda v: isinstance(v, bytes))
+        if env.PY3:
+            new_contract('unicode', lambda v: isinstance(v, unicode_class))
+
+if not contract:
+    # We aren't using real PyContracts, so just define a no-op decorator as a
+    # stunt double.
+    def contract(**unused):             # pylint: disable=function-redefined
+        """Dummy no-op implementation of `contract`."""
+        return lambda func: func
 
 
 def nice_pair(pair):